1. Field
The present invention relates to a composition comprising β-hydroxy-β-methylbutyrate (HMB) and Vitamin D, and methods of using a combination of HMB and Vitamin D to improve muscle mass, strength, or functionality.
2. Background
HMB
The only product of leucine metabolism is ketoisocaproate (KIC). A minor product of KIC metabolism is β-hydroxy-β-methylbutyrate (HMB). HMB has been found to be useful within the context of a variety of applications. Specifically, in U.S. Pat. No. 5,360,613 (Nissen), HMB is described as useful for reducing blood levels of total cholesterol and low-density lipoprotein cholesterol. In U.S. Pat. No. 5,348,979 (Nissen et al.), HMB is described as useful for promoting nitrogen retention in humans. U.S. Pat. No. 5,028,440 (Nissen) discusses the usefulness of HMB to increase lean tissue development in animals. Also, in U.S. Pat. No. 4,992,470 (Nissen), HMB is described as effective in enhancing the immune response of mammals. U.S. Pat. No. 6,031,000 (Nissen et al.) describes use of HMB and at least one amino acid to treat disease-associated wasting.
It has previous been observed that HMB alone or in combination with other amino acids is an effective supplement for restoring muscle strength and function in young athletes. Further, it has been observed that HMB in combination with two amino acids, glutamine and lysine, is effective in increasing muscle mass in elderly persons.
HMB is an active metabolite of the amino acid leucine. The use of HMB to suppress proteolysis originates from the observations that leucine has protein-sparing characteristics (1-4). The essential amino acid leucine can either be used for protein synthesis or transaminated to the α-ketoacid (α-ketoisocaproate, KIC)(1, 3). In one pathway, KIC can be oxidized to HMB. Approximately 5% of leucine oxidation proceeds via the second pathway (5). HMB is superior to leucine in enhancing muscle mass and strength. The optimal effects of HMB can be achieved at 3.0 grams per day, or 0.38 g/kg of body weight per day, while those of leucine require over 30.0 grams per day (3).
Once produced or ingested, HMB appears to have two fates. The first fate is simple excretion in urine. After HMB is fed, urine concentrations increase, resulting in an approximate 20-50% loss of HMB to urine (4, 6). Another fate relates to the activation of HMB to HMB-CoA (7-16). Once converted to HMB-CoA, further metabolism may occur, either dehydration of HMB-CoA to MC-CoA, or a direct conversion of HMB-CoA to HMG-CoA (17), which provides substrates for intracellular cholesterol synthesis. Several studies have shown that HMB is incorporated into the cholesterol synthetic pathway (12, 16, 18-20) and could be a source for new cell membranes that are used for the regeneration of damaged cell membranes (3). Human studies have shown that muscle damage following intense exercise, measured by elevated plasma CPK (creatine phosphokinase), is reduced with HMB supplementation within the first 48 hrs. The protective effect of HMB lasts up to three weeks with continued daily use (21-23).
In vitro studies in isolated rat muscle show that HMB is a potent inhibitor of muscle proteolysis (24) especially during periods of stress. These findings have been confirmed in humans; for example, HMB inhibits muscle proteolysis in subjects engaging in resistance training (4). The results have been duplicated in many studies (25) (21-23, 26-28).
The molecular mechanisms by which HMB decreases protein breakdown and increases protein synthesis have recently been reported (29-31, 31-33). In mice bearing the MAC16 cachexia-inducing tumor, HMB attenuated protein degradation through the down-regulation of key activators of the ubiquitin-proteasome pathway (30). Furthermore, HMB attenuated proteolysis-inducing factor (PIF) activation and increased gene expression of the ubiquitin-proteasome pathway in murine myotubes, thereby reducing protein degradation (31). PIF inhibits protein synthesis in murine myotubes by 50% and HMB attenuates this depression in protein synthesis (29). Eley et al demonstrated that HMB increases protein synthesis by a number of mechanisms, including the down-regulation of eukaryotic initiation factor 2 (eIF2) phosphorylation through an effect on dsRNA-dependant protein kinase (PKR) and upregulation of the mammalian target of rapamycin/70-kDa ribosomal S6 kinase (mTOR/p70S6k) pathway. The net result is increased phosphorylation of 4E-binding protein (4E-BP1) and an increase in the active eIF4G·eIF4E complex. Leucine shares many of these mechanisms with HMB, but HMB appears to be more potent in stimulating protein synthesis (29).
HMB can also increase protein synthesis by attenuating the common pathway that mediates the effects of other catabolic factors such as lipopolysaccharide (LPS), tumor necrosis factor-α/interferon-γ (TNF-α/IFN-γ), and angiotensin II (Ang II) (32, 33). HMB acts by attenuating the activation of caspases-3 and -8, and the subsequent attenuation of the activation of PKR and reactive oxygen species (ROS) formation via down-regulation of p38 mitogen activated protein kinase (p38MAPK). Increased ROS formation is known to induce protein degradation through the ubiquitin-proteasome pathway. HMB accomplishes this attenuation through the autophosphorylation PKR and the subsequent phosphorylation of eIF2α, and in part, through the activation of the mTOR pathway.
Numerous studies have shown an effective dose of HMB to be 3.0 grams per day as CaHMB (˜38 mg/kg body weight-day−1). This dosage increases muscle mass and strength gains associated with resistance training, while minimizing muscle damage associated with strenuous exercise (34) (4, 23, 26). HMB has been tested for safety, showing no side effects in healthy young or old adults (35-37). HMB in combination with L-arginine and L-glutamine has also been shown to be safe when supplemented to AIDS and cancer patients (38).
Studies in humans have also shown that dietary supplementation with 3 grams of CaHMB per day plus amino acids attenuates the loss of muscle mass in various conditions such as cancer and AIDS. (3, 4, 26, 34, 39, 40) A meta-analysis of supplements to increase lean mass and strength with weight training showed HMB to be one of only 2 dietary supplements that increase lean mass and strength with exercise (34). More recently it was shown that HMB and the amino acids arginine and lysine increased lean mass in a non-exercising, elderly population over a year-long study.
Vitamin D
Vitamin D has classically been associated with calcium and phosphorous metabolism and bone strength. Until recently, an adequate Vitamin D level has been defined using the Vitamin D deficiency disease rickets. While 1,25OH2-VitD3 is the active metabolite of Vitamin D, a measure of Vitamin D status widely accepted is serum (blood) circulating 25OH-VitD3. A circulating blood level between 10 and 15 ng 25OH-VitD3/mL will cause rickets in young children and has been accepted as the deficiency level for Vitamin D. Vitamin D can be synthesized by humans with adequate sun exposure or can be obtained through the diet and through supplements to the diet. Many factors influence the amount and effectiveness of Vitamin D found in the body. These factors include dietary intake, sun exposure, Vitamin D receptor number (VDR), conversion rate from cholecalciferol to 25OH-VitD3 and finally the conversion of 25OH-VitD3 to 1,25OH2-VitD3.
Most of the population in northern latitudes (most of the United States) do not produce Vitamin D in the winter regardless of sun exposure because the sun's ultraviolet B rays do not reach the earth during that time and therefore the only source of Vitamin D is dietary (42). As the 25 hydroxylation occurs in the liver and the 1 hydroxylation occurs primarily in the kidney, these two organs play a large role in determining the circulating levels of Vitamin D, and the functioning of these organs and thus Vitamin D status tends to decrease with age (42).
In a recent review, Holick details research showing that circulating levels of 25OH-VitD3 must reach as high as 30-40 ng/mL before parathyroid hormone (PTH) levels begin to plateau (43). Other researchers have found that increasing 25OH-VitD3 from 20 to 32 ng/mL increased intestinal calcium transport (44). Both of these criteria would point to a 25OH-VitD3 level of 30 ng/mL or greater being required for optimal regulation of calcium metabolism in the body. A recent review by Heaney describes the optimal level of 25OH-VitD3 to be 32 ng/mL or greater for optimal health which takes into account a number of aspects other than bone health and calcium metabolism (45). By these standards, from 40 to 100% of independent elderly men and women are Vitamin D deficient (43).
The 1-alpha, 25-Vitamin D hydroxylase in the kidney has been considered the primary source for synthesis of the circulating active metabolite of Vitamin D, 1,25OH2-VitD3. The activity of this enzyme is regulated on a whole body level by parathyroid hormone (PTH). Regulating 1,25OH2-VitD3 on a whole body level probably does not provide for optimal levels of the active vitamin for all body tissues at one time. Relatively recently tissue specific 1-alpha, 25-Vitamin D hydroxylases have been identified and are thought to mediate autocrine responses of Vitamin D at the tissue specific level (46, 47). Human vascular smooth muscle has 1-alpha, 25-Vitamin hydroxylase activity with a Km of 25 ng/mL. This means that the enzyme is operating at one half maximal capacity at a 25OH-VitD3 concentration of 25 ng/mL (48). Therefore serum levels of >25 ng/mL may be necessary for optimal active Vitamin D for vascular smooth muscle cells.
Muscle strength declines with age and a recently characterized deficiency symptom of Vitamin D is skeletal muscle weakness (43). Deficiency of Vitamin D and its hormonal effect on muscle mass and strength (sarcopenia) has been described as a risk factor in falls and bone fractures in the elderly (49). Loss of muscle strength has been correlated with a loss of Vitamin D receptors (VDR) in muscle cells (50). Supplemental Vitamin D of at least 800 IU per day may result in a clinically significant increase in VDR in muscle cells which may be in part be the mechanism whereby other studies have shown improvement in body-sway, muscle strength and falling risk were seen with Vitamin D supplementation at this level (51). While this muscular weakness associated with Vitamin D may not be surprising at classical Vitamin D deficiency levels (blood 25OH-VitD3 of <15 ng/mL), Bischoff-Ferrari et al continued to see improvement in lower extremity function up to and beyond 40 ng 25OH-VitD3/mL which are levels well above what previously might have been thought necessary for maximal benefit (52). This observation has been confirmed by other researchers that in fact minimal Vitamin D levels necessary to prevent rickets do not allow for maximal physical performance (53). A recent editorial in American Journal of Clinical Nutrition stated that all the literature available would indicate a 25OH-VitD3 level of at least 30 ng/mL is most optimal for health and disease (54).
While the exact mechanism is still unclear, it is clear that both the active metabolite, 1,25OH2-VitD3 and its precursor, 25OH-VitD3, play a significant role in normal functioning of muscle. Muscle contains VDRs for 1,25OH2-VitD3, found in both the nucleus and at the cell membrane (55-57) and these are also involved in non-specific binding 25OH-VitD3 as well (58). Studies by Haddad and Birge, published in the 1970s, show that feeding D3 to vitamin D deficient rats 7 hours prior to measurement increased protein synthesis as measured by 3H-leucine incorporation into muscle cell proteins. However, when the muscles were removed from the Vitamin D deficient rats and studied, only 25-OH Vit D3 acts directly in the muscles (58-60).
Early clinical evidence pointed to a reversible myopathy associated with Vitamin D deficiency (61). Vitamin D receptors were discovered in muscle tissue, thus providing direct evidence of Vitamin D's effect on muscle function (51, 62). Muscle biopsies in adults with Vitamin D deficiency exhibit mainly type II muscle fiber atrophy (63). Type II fibers are important because they are the first initiated to prevent a fall. A recent randomized controlled study found that daily supplementation of 1,000 IU of Vitamin D2 in elderly stroke survivors resulted in an increase in mean type II fiber diameter and in percentage of type II fibers (64). There was also a correlation between serum 25OH-VitD3 level and type II fiber diameter.
Vitamin D conveys its action by binding to VDR. VDR is expressed in particular stages of differentiation from myoblast to myotubes (55, 65, 66). Two different VDRs have been described. One is located at the nucleus and acts as a nuclear receptor and the other is located at the cell membrane and acts as a cellular receptor. VDR knockout mice are characterized by a reduction in both body weight and size as well as impaired motor coordination (67). The nuclear VDR is a ligand-dependent nuclear transcription factor that belongs to the steroid-thyroid hormone receptor gene superfamily (68). Bischoff et al (69) reported the first in situ detection of VDR in human muscle tissue with significant associated intranuclear staining for VDR. Once 1,25OH2-VitD3 binds to its nuclear receptor, it causes changes in mRNA transcription and subsequent protein synthesis (70). The genomic pathway has been known to influence muscle calcium uptake, phosphate transport across the cell membrane, phospholipid metabolism, and muscle cell proliferation and differentiation. 1,25OH-VitD3 regulates muscle calcium uptake by modulating the activity of calcium pumps in sacroplasmic reticulum and sacrolemma (61). Modifications of calcium levels impact muscle function (71). In vitro experiments support these findings by demonstrating an increased uptake of 45Ca in cells exposed to physiological levels of 1,25OH2-VitD3 (72). The calcium binding protein calbindin D-9K is synthesized as a result of activation of nuclear VDR (62). 1,25OH2-VitD3 plays a role in regulating phosphate metabolism in myoblasts by accelerating phosphate uptake and accumulation in cells. 1,25OH2-VitD3 acts rapidly, presumably through cell membrane VDRs (56, 57). While binding to these receptors, there is an activation of second-messenger pathways (G-proteins, cAMP, inositol triphosphate, arachidonic acid) (73-75) or the phosphorylation of intracellular proteins. These would in turn activate protein kinase C (PKC), leading to release of calcium into muscle cells, and ultimately resulting in active transport of Ca into the sacroplasmic reticulum by Ca-ATPase. This process is important for muscle contraction. Additionally, PKC affects enhancements of protein synthesis in muscle cells (76). Recent data (77) indicate that 1,25OH-VitD3 has a fast activation of mitogen-activated protein kinase (MAPK) signaling pathways, which in turn forward signals to their intracellular targets that effect the initiation of myogenesis, cell proliferation, differentiation, or apoptosis.
Vitamin D may also regulate formation and regeneration of tight junctions and neuromuscular junctions. In vitro studies that found that Vitamin D regulates expression of VDR and the neural growth factor (NGF) in Schwann cells (78). Recent studies have shown that Vitamin D enhances glial cell line-derived neurotrophic factor (GDNF) in rats and that this may have beneficial effects in neurodegenerative disease (79). Therefore, Vitamin D could act through several mechanisms of cellular function and neural interaction to improve overall muscle strength and function.
A need exists for a composition and methods to increase muscle mass and improve function and strength. The present invention comprises a composition and methods of using a combination of Vitamin D and HMB that results in such an increase in muscle mass and improvement in strength and function.